Modern comprehensive monitoring systems for power transformers lead to increased operating reliability and reduction of operating and life-cycle costs. The monitoring of all transformer main components is one requirement to achieve the knowledge of the operation conditions of the transformer. A further improvement is achieved by correlation of all measured and analyzed values of the transformer or even across transformers in substation. In order to meet these requirements, specific architectures and software models are essential. This contribution describes the application of the above mentioned requirements in comprehensive monitoring systems. More than 100 years of experience in the manufacturing and service of power transformers and the knowledge gained from monitoring of several hundred transformers made it possible, to create a transformer expert system, which features complete data management, analysis algorithms and diagnosis functions. The simple and interactive user interface is an essential element for the application of the monitoring system and the built in simulator. With the use of the comprehensive monitoring system and the derived optimized operation, the lifetime of the transformer can be extended. The knowledge of the condition of the transformer and the early recognition of incipient failures with the help of the monitoring system ensure the safe and enduring operation of the power transformer.